Studies on a range of prokaryote and eukaryote cells, tissues and whole organisms have shown that a newly synthesized DNA/RNA-lipoprotein complex is released from living cells but not from dying or dead cells (1-8). DNA-dependent DNA polymerase and DNA-dependent RNA polymerase are present amongst the proteins (9-14). The complex is a novel cytosolic component of eukaryote cells that is released in a regulated manner and has been named the virtosome (15).
The DNA of the virtosome (3-5×105 daltons) forms ca 5% of the total virtosomal complex and is synthesised in the nucleus, possibly involving the DNA synthetic system described for extra-chromosomal circular DNA (16). The DNA appears to be similar in both cytosolic and released virtosomes. It is released into the cytosol where it appears ca 3 h later (17). Due to the size of the DNA, it may not leave the nucleus via the nuclear pores but may adopt an alternative route proposed for mRNP nuclear export (18). In the cytosol, the DNA combines with newly synthesised proteins, lipids and RNA prior to being released from the cell 3-5 h later (7, 8, 17, 19, 20). The RNA forms about half of the virtosomal components with protein forming about 40% (21). Experiments with chick embryo fibroblast (CEF) virtosomes released into the culture medium and allowed to enter fresh CEF cells showed that the released and the re-entered virtosomes isolated from the cytosol of the recipient cells were collected at the same point after agarose gel chromatography (22). This implied that membrane was neither gained nor lost on either exit from or entry to cells and that there was not a classical membrane associated with the virtosomes (7, 8, 17, 19, 20). This was confirmed by the phospholipid analyses of virtosomes showing them to have phosphatidylcholine levels too low for the presence of a standard membrane (21).
The released virtosome acts as an intercellular messenger that readily enters other cells where it can modify the biology of the recipient cells (15). Such modifications include immunological changes (23,24), the transformation of normal cells into tumour cells (25) and inhibition of DNA synthesis (26, 27). Virtosomes are able to enter cells without being digested by the lysosomal system. It is not clear how this is achieved. It could be by inhibiting the modification of lysosomal pH as has already been demonstrated for other systems (28), or it could bypass the lysosomal system as has been demonstrated for other DNA structures. Here it is possible that the virtosomal phosphatidylinositol plays a role (28-31).
The transformation of normal cells into tumour cells via released tumour DNA sequences has been demonstrated (25). Thus, the SW480 cell line, originating from a human colon carcinoma, presented a point mutation of the K-ras gene on both alleles. These cells in culture released the DNA-complex containing the mutated K-ras gene (Kirsten rat sarcoma viral oncogene homologue). When crude SW480 cell supernatant was given to NIH-3T3 cells, without the addition of any other compound, transformed foci appeared as numerously as those occurring after a transfection provoked by a clonal K-ras gene administered as a calcium precipitate. The presence of a mutated K-ras gene in the transfected foci of the NIH-3T3 cells was checked by hybridization after PCR. This result was confirmed by sequencing the PCR product (25).
Once a tumour has been detected it is important to be able to (i) block the further growth of the tumour, (ii) reduce tumour size and (iii) prevent the development of possible metastases. (i) and (iii) are currently treated either directly by either radiotherapy or chemotherapy or by surgery followed by chemotherapy/radiotherapy.
Normally, the virtosomes act primarily within a given tissue within the whole organism. Thus, such virtosomes released from non-dividing cells e.g. hepatocytes within the liver, will be unlikely to interact directly with tumour cells.